In recent years, as a lighting lamp, an LED lighting device that uses an LED as a light source has come into widespread use. Until now, in the case where the LED is used for a purpose other than a lighting lamp, it is common to light the LED by a DC power source, and light control of the LED is performed by digital signal processing, for example, by changing the duty ratio or the number of pulses.
However, the LED lighting device is supposed to be used as a replacement of an incandescent bulb or a fluorescent lamp, and therefore it is desirable to enable lighting by utilizing the commercial alternating-current power source. Further, it is desired for the LED lighting device using the LED to have the light control function.
There is known an LED lighting device that lights the LED by directly applying a full-wave rectified waveform obtained from the commercial alternating-current power source to the LED string. The LED string is formed by connecting a plurality of LEDs in series and designed so as to be capable of resisting a high voltage. Compared to the LED lighting device adopting another system in which the LED is lit by generating a fixed voltage from the commercial alternating-current power source, the LED lighting device in which the full-wave rectified waveform is applied directly to the LED string is characterized in that the circuit configuration is simple and compact.
However, if an undulating voltage is applied to the LED string, the LED lights up only for the period of time during which the undulating voltage exceeds a threshold value of the LED string. For example, in the case where a forward voltage Vf of the LED is 3 V and 40 LEDs are connected in series in the LED string, the threshold value of the LED string will be 120 V. Consequently, when the effective value of the commercial alternating-current power source is 100 V, in the LED lighting device, the LED lights up only for a short period of time during which the undulating voltage exceeds 120 V. Thus, if the undulating voltage is applied to the LED string, the power factor and the distortion factor deteriorate, as well as the LED lighting device becoming dark and flicker becoming conspicuous.
In order to address this, as a method for lengthening the lighting period, there is known a method in which the LED string is divided into a plurality of LED strings and in the phase in which the voltage of the undulating voltage is low, only part of the LED strings is lit, and in the phase in which the voltage of the undulating voltage is high, the number of LED strings to be lit is increased. The number of LED strings to be lit is adjusted by a bypass circuit connected to a connection point of the LED strings. The bypass circuit is turned on (brought into conduction) in the low voltage phase of the undulating voltage and turned off (brought out of conduction) in the high voltage phase of the undulating voltage. Turning on/off of the bypass circuit is controlled in accordance with the voltage of the undulating voltage or the current value of a current flowing through the LED string.
FIG. 26 of Patent Document 1 is illustrated in FIG. 1 as an example of the LED lighting device that controls the bypass circuit in accordance with the current value of a current flowing through the LED string.
In FIG. 1, FIG. 26 of Patent Document 1 is redrawn so as not to deviate from the gist of FIG. 26. As illustrated in FIG. 1, the LED lighting device 900 has the bridge rectifier 905, the first and second LED strings 910 and 930, the bypass circuit 920, and the current limit resistor 933. The commercial alternating-current power source 906 is connected to the input terminal of the bridge rectifier 905.
The bridge rectifier 905 has the four diodes 901, 902, 903, and 904, and the terminal A is the output terminal of the full-wave rectified waveform and the terminal B is the terminal from which the reference voltage is given. Within the first LED string 910, a large number of LEDs including the LEDs 911 and 912 are connected in series and within the second LED string 930, a large number of LEDs including the LEDs 931 and 932 are connected in series. The bypass circuit 920 has the pull-up resistor 921, the bypass resistor 924, the field effect transistor 922 (hereinafter also referred to as the FET), and the bipolar transistor 923 (hereinafter also referred to as the transistor). The bypass circuit 920 further has the bypass first input terminal 927, the bypass second input terminal 928, and the bypass output terminal 929. The bypass first input terminal 927 is connected to the cathode of the LED in the final stage of the first LED string 910 (hereinafter also referred to as the cathode of the first LED string 910) and to the anode of the LED in the initial stage of the second LED string 930 (hereinafter also referred to as the anode of the second LED string 930). The bypass second input terminal 928 is connected to the cathode of the LED in the final stage of the second LED string 930 (hereinafter also referred to as the cathode of the second LED string 930) via the current limit resistor 933. The FET 929 is an enhancement type nMOS-FET.
With reference to FIG. 2, the operation of the LED lighting device 900 is explained. FIG. 2A is a diagram illustrating part of the output signal of the bridge rectifier 905 and specifically, is a diagram illustrating the full-wave rectified waveform voltage of the output signal of the bridge rectifier 905. FIG. 2B is a diagram illustrating the waveform of the current I flowing through the LED lighting device 900. The horizontal axis in FIGS. 2A and 2B represents time, the vertical axis in FIG. 2A represents the voltage value, and the vertical axis in FIG. 2B represents the current value. The time represented by the horizontal axis in FIG. 2A is identical to the time represented by the horizontal axis in FIG. 2B. During the period of time t1, the output voltage of the bridge rectifier 905 does not reach the threshold voltage Vth1 of the first LED string 910, and therefore the circuit current I does not flow.
During the period of time t2 in the low voltage phase of the output voltage of the bridge rectifier 905, the output voltage of the bridge rectifier 905 exceeds the threshold voltage Vth1 of the first LED string 910, however, does not exceed the total threshold voltage of the threshold voltage Vth1 of the first LED string 910 and the threshold voltage Vth2 of the second LED string 930. During the period of time t2, the circuit current I returns to the bridge rectifier 905 via the bypass circuit 920. During the period of time t2, feedback is applied so that the base-emitter voltage of the transistor 923 is kept at 0.6 V and the bypass circuit 920 performs the constant current operation.
Next, during the last short period of time of the period of time t2, the output voltage of the bridge rectifier 905 becomes greater than the total threshold voltage of the threshold voltage Vth1 of the first LED string 910 and the threshold voltage Vth2 of the second LED string 930 and a current begins to flow through the bypass second input terminal 928 via the second LED string 930.
Next, during the period of time t3, the output voltage of the bridge rectifier 905 exceeds the total threshold voltage of the threshold voltage Vth1 of the first LED string 910 and the threshold voltage Vth2 of the second LED string 930 and a current flows through the bypass second input terminal 928 via the first and second LED strings 910 and 930. When a current flows through the bypass second input terminal 928, the transistor 923 is saturated, the gate-source voltage of the FET 922 becomes 0 or negative, and the FET 922 is turned off. When the FET 922 is turned off, the current input from the bypass first input terminal 927 of the bypass circuit 920 is only a minute current via the pull-up resistor 921 having a high resistance value and most of the current I flows via the first and second LED strings 910 and 930. During the period of time during which the output voltage of the bridge rectifier 905 decreases, the operation is performed sequentially in the opposite order of the operation during the period of time during which the voltage of the full-wave rectified waveform increases.
As described above, the conventional LED lighting device 900 illustrated in FIG. 1 controls the turning on and off of the bypass circuit 920 by the current flowing through the LED string, and therefore is characterized in that the circuit scale is reduced and noise is small, since the circuit current I changes smoothly. However, the conventional LED lighting device 900 illustrated in FIG. 1 has a problem in that a function to adjust the amount of emission of the LED string according to the use environment, i.e. the light control function is not provided.
On the other hand, it is known that light control of an incandescent bulb is performed using the TRIAC (registered trademark) dimmer. However, the alternating-current waveform output from the TRIAC (registered trademark) dimmer is a waveform part of which is lost, and therefore, in the case where the LED lighting device is lit, there is a possibility that flicker will be conspicuous.
Thus, Patent Document 2 describes that the rectifier configured to convert the AC power source into the DC power source is provided and the LED is lit by the DC voltage and light control thereof is performed. Specifically, Patent Document 2 describes the LED lighting device that performs light control by changing the voltage of the AC power source supplied to the LED lighting device (e.g., see FIG. 6 of Patent Document 2). Further, Patent Document 2 describes the LED light controller that performs light control by various kinds of digital control, such as pulse modulation and pulse width modulation, by using the processor-based controller (e.g., see FIG. 7 of Patent Document 2). The LED lighting device including the LED light controller described in Patent Document 2 enables lighting with inconspicuous flicker and light control.
However, the LED lighting device including the LED light controller described in Patent Document 2 needs to have an excellent DC power source and has a problem in that the light control circuit becomes complicated. In order to simplify the rectifier circuit, it is desirable to make it possible to light the LED by directly applying the undulating voltage in the full-wave rectified waveform etc. obtained from the commercial alternating-current power source to the LED.
In the case where light control of an incandescent lamp is performed by using the TRIAC (registered trademark) dimmer, the TRIAC (registered trademark) dimmer is embedded in the wall in many cases and additional construction to embed the TRIAC (registered trademark) dimmer in the wall is necessary, and therefore this is sometimes inconvenient. Thus, a method for setting the amount of light control without using the TRIAC (registered trademark) dimmer has been proposed and in this method, for example, ON/OFF control of a wall switch is utilized (e.g., see Patent Document 3).
FIG. 1 of Patent Document 3 illustrates the lighting device including the inverter circuit 1 that lights the lamp load L, the inverter control circuit 4, the power source shut-off detection circuit 2, and the time determination circuit 3. The inverter control circuit 4 changes the lighting state of the lamp load L by controlling the operation of the inverter circuit 1. The power source shut-off detection circuit 2 detects shut-off of the power source by the operation of the switch SW1. The time determination circuit 3 determines the time during which the power source is shut off by the power source shut-off time detection signal of the power source shut-off detection circuit 2 and selects the lighting state of the lamp load L by controlling the inverter control circuit 4 in the case where the time is equal or within a predetermined time set in advance. However, the control described in Patent Document 3 relates to lighting of the incandescent lamp, and the technique related to lighting of the LED is not described at all. Further, the lighting device described in Patent Document 3 uses an inverter circuit, and therefore there is a problem in that the inverter circuit is large and expensive.
FIG. 7 of Patent Document 4 illustrates the LED lighting device, for which light control can be performed, including the bridge rectifier 102, the toggle detector 74, the maintenance voltage supply circuit 71, the counter 96, and the LED lighting driver 80. The bridge rectifier 102 rectifies the AC voltage applied via the wall switch and provides a DC voltage. The toggle detector 74 monitors the toggle operation of the wall switch 98. The maintenance voltage supply circuit 71 provides a maintenance voltage and the counter 96 counts the toggle operation. The LED lighting driver 80 performs light control of the LED light source at multiple levels based on the counted value.
In the configuration described in Patent Document 4, pulse width modulation is performed after converting the output of the bridge rectifier 102 into a direct-current voltage with less ripple. In order to generate a direct-current voltage with less ripple necessary for pulse width modulation, an electrolytic capacitor having a high withstand voltage and a large capacitance is necessary. However, in addition to the size of the electrolytic capacitor being large, the life of the electrolytic capacitor becomes short in an environment of high temperature, such as in the LED lighting device. Further, in the LED lighting driver 80, almost all the components are turned into an integrated circuit. However, a variety of circuits, such as an oscillator circuit, are incorporated, and therefore the circuit configuration tends to become complicated.
Patent Document 1: WO 2011020007 A1
Patent Document 2: JP-2005-524960-A
Patent Document 3: JP-H4-115799-U
Patent Document 4: JP-2011-103285-A